This invention relates to gold mining, and more particularly to the recovery of gold fines from low grade ores.
It is recognized that conventional mechanical processing of gold ores results in the loss of 10% or more of the gold content of the ore, because it is of such fine size that it does not respond to recovery by the procedures used to recover the larger size particulate gold.
Various mechanical procedures have been used and proposed for the purpose of recovering fine gold. One is the use of a gold pan. This method uses gravity, agitation and flotation to cause the heavier gold particles to sink to the bottom of the pan while lighter materials are caused to flow out of the pan. The pan limits the amount of material that can be handled as the process of separating the tails by density using a pan is a tedious one. In addition, as the content of heavy materials, such as the black, magnetically susceptible mineral magnetite, increases, the loss of fine gold also increases due to inter-particular collisions between the gold fines and this heavy mineral which cause the gold to remain mixed with the black sand being washed from the pan.
Another method is the use of a sluice. Traditional sluices generally are wooden troughs constructed with cross bars or riffles which allow the gold to be trapped against the riffles. However, a decrease in water flow from optimum can slow the passage of ore or increases in water flow can cause turbulence which causes the gold previously trapped to be reentrained in the flow and lost. To minimize these effects, sluice boxes have been modified by placing a mat of carpet or synthetic fiber beneath the riffles to help hold the gold that has been trapped. However, this requires the disassembling of the sluice in order to remove and recover the trapped gold from the mat.
Still another method of separating fine gold from ore is the use of a cast iron spiral fitted with magnets, as described in U.S. Pat. No. 5,205,414. This method effects separation by adding a heavy magnetic material such as magnetite to the ore mixture which concentrates in the separator with the gold and is then removed from the ore mixture by the use of magnets. This method requires the use of large quantities of magnetite which requires recycling. In addition, complex mechanical systems are necessary to use this method, thus driving up operating costs.
Another method proposed for separating fine gold from ore is the use of a continuously moving endless belt fitted with special riffle pockets, as disclosed in U.S. Pat. No. 5,033,332. Such equipment requires continual maintenance and frequent repair. In addition, scaling up this method is problematical.
Chemical methods, such as cyanide extraction and mercury amalgamation also are employed for large scale recoveries. However, they are environmentally and physically hazardous procedures.